var xmlHttp;
function linkServer(){
    xmlHttp=GetXmlHttpObject();
    if (xmlHttp==null){
     alert ("Your browser does not support AJAX!");
     return;
     }
    var url="login"+"?"+"username="+document.getElementById("username").value+
    "&"+"password="+document.getElementById("password").value;

    xmlHttp.open("GET",url,true);
    xmlHttp.onreadystatechange=stateChanged;
    xmlHttp.send(null);
}
function stateChanged() {

  if (xmlHttp.readyState==4){
  alert("xmlState:"+xmlHttp.status);
   alert("xmlHttp.responseText:"+xmlHttp.responseText);
     if(xmlHttp.responseText=="DONE"){
     document.getElementById("user").innerHTML="(登录成功)";
  }
else
    document.getElementById("user").innerHTML="(用户名或密码错误)";
}
}

//获得HTTP对象
function GetXmlHttpObject(){
  var xmlHttp=null;
  try{
     xmlHttp=new XMLHttpRequest();
      }catch (e){
       try{
     xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
        }catch (e)
       {
     xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
  }
return xmlHttp;
}

package com.service;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class Login extends HttpServlet {
    private static final String CONTENT_TYPE = "text/html; charset=GBK";

    //Process the HTTP Get request
    public void doGet(HttpServletRequest request, HttpServletResponse response) throws
            ServletException, IOException {
        String u1 = request.getParameter("username");
        String p1 = request.getParameter("password");
        System.out.println("u1:" + u1);
        System.out.println("u2:" + p1);

        response.setContentType(CONTENT_TYPE);
        PrintWriter out = response.getWriter();
//下面就可以数据库查询，这里简单的作个判断
        if((u1!=null&&u1.equals("hello"))&&((p1!=null&&p1.equals("123456"))))
        out.print("DONE");
    else{
        out.print("FAIL");
    }
        out.close();

    }

    //Process the HTTP Post request
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws
            ServletException, IOException {
        doGet(request, response);

    }
}